When tightening down your axle nut, it's easy to inadvertently compress the interior bearing spacer. This puts stress on your bearings and introduces slop into your setup, which detracts from the precision a bearing spacer is intended to provide.
The Atlas Blackout longboard bearings have a built-in, flared spacer that increases the surface area of the load bearing foot between the two bearings. This prevents compression of the spacer, which means a more precise ride, faster roll speed and longer life.
ABEC 9 - The ABEC scale is essentially a measurement of bearing tolerance for high precision bearings as dictated by the Annular Bearing Engineering Committee (ABEC). The scale runs from ABEC 1 through ABEC 9 using odd integers only. Essentially, the higher up on the scale a bearing is rated the tighter the tolerance is.
Tighter bearing tolerance means that the bearing moves with less resistance at high speeds, giving you a bit more roll for your push. In general most ABEC 3 or ABEC 5 bearings have steel races (shell of the bearing) with hardened steel ball bearings, and most ABEC 7 or ABEC 9 bearings feature steel races with super hard (much harder than steel) ceramic or titanium bearings.
Built-in Spacers - Built-in spacers allow you to tighten the axle all the way to ensure the bearings stay parallel in the wheel.